Chemical vapor deposition techniques utilize a variety of different reactant gases. The gases are brought together inside of a reaction chamber in the presence of a substrate, and the gases react and deposit a film on the surface of the substrate. Some of the gases utilized for CVD techniques originate as liquids and must be evaporated and transported in their gaseous state to the reaction chamber. The gas delivery systems and lines for transporting the necessary reactant gases to the reaction chamber are heated to prevent condensation of the gas in the lines.
As one example, titanium tetrachloride (TiCl.sub.4) is utilized as a reactant gas in chemical vapor deposition reactions to deposit titanium-containing film layers onto the surface of a substrate. A source of titanium tetrachloride in liquid form is heated to provide a vapor pressure in a system containing a gas delivery line and the pressure drives the gas through the gas delivery line. A mass flow controller controls the rate of gas flow at a level appropriate for the vapor deposition reaction. Mass flow controllers control and measure the amount of gas delivered through the system typically in standard cubic centimeters per minute (sccm). To provide a TiCl.sub.4 delivery pressure of approximately 100 Torr in a typical reactant gas delivery systems, a liquid source of TiCl.sub.4 is heated to around 70.degree. C. To maintain the pressure and prevent condensation, the gas delivery lines and any in-line components, such as valves and connectors, must be kept above 70.degree. C. However, the maximum operating temperature for commercially available mass flow controllers is around 90.degree. C. Therefore, while the gas delivery lines and components must be maintained at a minimum temperature, that temperature cannot exceed a predetermined maximum due to the constraints of the mass flow controller.
Traditionally, heating the gas delivery lines in a CVD reactant gas system has involved wrapping the lines and any other in-line gas components, such as the mass flow controller, with heater tape. Heater tape incorporates an electrically-powered resistive heating element which generates heat and conductively transfers it to the gas delivery line and components. A drawback of heater tape, however, is that the wrapping process is very tedious and often sloppy due to the various sizes and shapes of the components which must be heated. Further, the discontinuous connections between the gas line and in-line components hinders sufficient coverage of the system. The sloppiness of the wrapping procedure and the non-uniform shape of the resulting heater tape creates temperature non-uniformity throughout the heated line. Additionally, the different masses and heat absorption characteristics of the components further hinders temperature uniformity. The result is condensation of TiCl.sub.4 in the line, which leads to undesirable deposition process variability, and may ultimately lead to failure of the in-line components, such as, the mass flow controller.
Additional heating difficulties result when heater tape is used because of the structure of the line connected between the mass flow controller and the reaction chamber. Traditional gas delivery lines have included a flexible line portion leading into the reaction chamber so that the chamber cover may be lifted away from the chamber. The flexibility of the flexible line portion causes the heater tape to shift when the portion flexes, further contributing to poor temperature uniformity.
Still further, prior art gas delivery systems have not addressed the problem of chemical reaction within the gas line itself. Specifically, ammonia gas (NH.sub.3) is often utilized with TiCl.sub.4 to deposit titanium-containing films by CVD processes. Depending upon the process parameters, an amount of the ammonia gas introduced into the reaction chamber may be drawn back into the TiCl.sub.4 delivery line. The reaction product of NH.sub.3 and TiCl.sub.4 is a yellow, powdery adduct. The adduct forms within the line and coats the walls of the line, ultimately interrupting the flow of TiCl.sub.4 and also producing contaminants within the line and the reaction chamber. Generally, prior art gas delivery systems do not adequately address such adduct formation, and therefore require costly and time-consuming steps such as regular cleaning of the gas delivery line.
Accordingly, it is an objective of the present invention to provide an improved reactant gas delivery system which eliminates the drawbacks of prior art systems. Specifically, it is an objective of the present invention to provide uniform heating of the reactant gas line and reactant gas while eliminating the heating drawbacks of various prior art systems, such as those utilizing heater tape. Further, it is an objective of the present invention to ensure temperature uniformity in the entire gas delivery line from the source to the reaction chamber to prevent condensation. It is further an objective to provide temperature uniformity while preventing damage to the mass flow controller and any other temperature sensitive components connected in-line in the gas delivery system. It is still further an objective of the present invention to adequately prevent formation of an adduct or other reaction product within the gas delivery line.